Integrated micro combined heat and power system

ABSTRACT

An integrated system to provide both heat and electric power. The integrated, or cogeneration, system operates with an organic working fluid that circulates in a Rankine-type cycle, where the organic working fluid is superheated by a heat source, expanded through an involute spiral wrap (scroll) expander such that the organic working fluid remains superheated through the expander, cooled in a condenser, and pressurized by a pump. Heat exchange loops within the system define hot water production capability for use in space heating and domestic hot water, while the generator is coupled to the scroll expander to generate electricity.

[0001] This application is a Divisional of U.S. patent application Ser.No. 09/998,705 filed Nov. 30, 2001 (non allowed), which claims thebenefit of U.S. Provisional Application No. 60/311,514 filed Aug. 10,2001.

BACKGROUND OF THE INVENTION

[0002] The present invention generally relates to a cogeneration systemfor the supply of electrical power, space heating (SH) water anddomestic hot water (DHW), and more particularly to a small scaleRankine-type cogeneration system that utilizes a scroll expander and anorganic working fluid.

[0003] The concept of cogeneration, or combined heat and power (CHP),has been known for some time as a way to improve overall efficiency inenergy production systems. With a typical CHP system, heat (usually inthe form of hot air or water) and electricity are the two forms ofenergy that are generated. In such a system, the heat produced from acombustion process can drive an electric generator, as well as heat upwater, often turning it into steam for dwelling or process heat. Mostpresent-day CHP systems tend to be rather large, producing heat andpower for either a vast number of consumers or large industrialconcerns. Traditionally, the economies of scale have prevented such anapproach from being extrapolated down to a single or discreet number ofusers. However, increases in fuel costs have diminished the benefits ofcentrally-generated power. Accordingly, there is a potentially greatmarket where large numbers of relatively autonomous, distributedproducers of heat and electricity can be utilized. For example, inolder, existing heat transport infrastructure, where the presence offluid-carrying pipes is pervasive, the inclusion of a system that canprovide CHP would be especially promising, as no disturbance of theadjacent building structure to insert new piping is required. Similarly,a CHP system's inherent multifunction capability can reduce structuralredundancy.

[0004] The market for localized heat generation capability in Europe andthe United Kingdom (UK), as well as certain parts of the United States,dictates that a single unit for single-family residential and smallcommercial sites provide heat for both SH (such as a hydronic systemwith radiator), and DHW (such as a shower head or faucet in a sink orbathtub), via demand or instantaneous system. Existing combination unitsare sometimes used, where heat for DHW is accumulated in a combinationstorage tank and boiler coil. In one configuration, SH water circulatesthrough the boiler coil, which acts as the heating element for the waterin the storage tank. By way of example, since the storage capacityrequired for instantaneous DHW supplying one to two showers in a singlefamily residence (such as a detached house or a large apartment) isapproximately 120 to 180 liters (roughly 30 to 50 gallons), it followsthat the size of the storage tank needs to be fairly large, sometimesprohibitively so to satisfy thermal requirements of up to 25 kilowattsthermal (kW_(t)) for stored hot water to meet such a peak shower demand.However, in newer and smaller homes there is often inadequate room toaccommodate such storage tank volume. In addition to the need forinstantaneous DHW capacity of up to 25 kW_(t), up to 10 kW_(t) for SH isseasonally needed to heat an average-sized dwelling.

[0005] Furthermore, even in systems that employ SH and DHW into a singleheating system to consolidate spacing, no provision for CHP is included.In the example given above, it is likely that the electricalrequirements concomitant with the use of 35 kW_(t) will be between 3 and5 kilowatts electric (kW_(e)). The traditional approach to providingboth forms of power, as previously discussed, was to have a largecentral electricity generating station provide electricity on a commongrid to thousands or even millions of users, with heat and hot waterproduction capacity provided at or near the end-user on an individual orsmall group basis. Thus, with the traditional approach, the consumer hasnot only little control over the cost of power generation, as such costis subject to prevailing rates and demand from other consumers, but alsopays more due to the inherent inefficiency of a system that does notexploit the synergism of using otherwise waste heat to provide eitheradditional electric generation or heating capacity.

[0006] Large-scale (in the megawatt (MW) range and up) cogenerationsystems, while helpful in reducing the aforementioned inefficiencies ofcentrally-based power generation facilities, are not well-suited toproviding small-scale (below a few hundred kW) heat and power,especially in the small-scale range of a few kW_(e) and below(micro-based systems) to a few dozen kW_(e) (mini-based systems). Muchof this is due to the inability of the large prime mover systems toscale down, as reasonable electrical efficiency is often only achievedwith varying load-responsive systems, tighter dimensional tolerances ofkey components and attendant high capital cost. Representative of thisclass are gas turbines, which are expensive to build for small-scaleapplications, and sacrifice efficiency when operating over varyingelectrical load requirements. Efficiency-enhancing devices, such asrecuperators, tend to reduce heat available to the DHW or SH loops, thuslimiting their use in high heat-to-power ratio (hereinafter Q/P)applications. A subclass of the gas turbine-based prime mover is themicroturbine, which includes a high-speed generator coupled to powerelectronics, could be a feasible approach to small-scale cogenerationsystems. Other shortcomings associated with large-scale CHP systems stemfrom life-limited configurations that have high maintenance costs. Thisclass includes prime movers incorporating conventional internalcombustion engines, where noise, exhaust emissions, lubricating oil andspark plug changes and related maintenance and packaging requirementsrender use within a residential or light commercial dwellingobjectionable. This class of prime mover also does not reject asufficient amount of heat for situations requiring a high QIP, such asmay be expected to be encountered in a single family dwelling. Otherprime mover configurations, such as steam turbines, while generallyconducive to high Q/P, are even less adapted to fluctuating electricalrequirements than gas turbines. In addition, the steam-based approachtypically involves slow system start-up, and high initial system cost,both militating against small-scale applications.

[0007] In view of the limitations of the existing art, the inventors ofthe present invention have discovered that what is needed is anautonomous system that integrates electric and heat production into anaffordable, compact, efficient and distributed power generator.

BRIEF SUMMARY OF THE INVENTION

[0008] These needs are met by the present invention, where a newmicro-CHP system is described. In micro-CHP, a compact prime mover canprovide both electric output, such as from a generator coupled to a heatsource, as well as heat output to provide warm air and hot water todwellings. What distinguishes micro-CHP from traditional CHP is size: inthe micro-CHP, electric output is fairly small, in the low kW_(e) oreven sub-kW_(e) range. The system of the present invention can providerapid response to DHW requirements, as the size of tanks needed to storewater are greatly reduced, or possibly even eliminated. The size of themicro-CHP system described herein can be adapted to particular userneeds; for example, a system for a single-family dwelling could be sizedto produce approximately 3 to 5 kW_(e), 10 kW_(t) SH and 25 kW_(t) DHW.For small commercial applications or multi-dwelling (such as a group ofapartment units) use, the system could be scaled upwards accordingly.The heat to power ratio, Q/P, is an important parameter in configuringthe system. For most residential and small commercial applications, aQ/P in the range of 7:1 to 11:1 is preferable, as ratios much lower thanthat could result in wasted electrical generation, and ratios muchhigher than that are not practical for all but the coldest climates(where the need for heating is more constant than seasonal). Since theproduction of electricity (through, for example, a generator or fuelcell) is a byproduct of the prime mover heat generation process, noadditional carbon dioxide and related atmospheric pollutants aregenerated, thus making the system of the present invention amenable tostricter emission control requirements.

[0009] According to a first aspect of the present invention, acogeneration system configured to operate with an organic working fluidis disclosed. The system includes a heat source, a first circuitconfigured to transport the organic working fluid, and a generatoroperatively coupled to a scroll expander to produce electricity. Thefirst circuit includes a scroll expander configured to receive theorganic working fluid, a condenser in fluid communication with thescroll expander, and a pump configured to circulate the organic workingfluid. The first circuit is in thermal communication with the heatsource such that heat transferred therefrom converts the organic workingfluid to a superheated vapor. The use of organic working fluid, ratherthan a more readily-available fluid (such as water) is important whereshipping and even some end uses could subject portions of the system tofreezing (below 32° Fahrenheit). With a water-filled system, damage andinoperability could ensue after prolonged exposure to sub-freezingtemperatures. In addition, by using an organic working fluid rather thanwater, corrosion issues germane to water in the presence of oxygen, andexpander sizing or staging issues associated with low vapor densityfluids, are avoided. The organic working fluid is preferably either ahalocarbon refrigerant or a naturally-occurring hydrocarbon. Examples ofthe former include R-245fa, while examples of the latter include some ofthe alkanes, such as isopentane. Other known working fluids andrefrigerants, despite exhibiting attractive thermodynamic properties,are precluded for other reasons. For example, R-11 is one of a class ofrefrigerants now banned in most of the world for environmental reasons.Similarly, R-123, much less environmentally objectionable (for now) thanR-11, is the subject of decomposition concerns under certain micro-CHPoperating conditions. The need to operate the condenser at a high enoughtemperature to allow useful hydronic space heating and the need to havea substantial vapor expansion ratio (of 5 to 7 or 8) limits the numberof fluids with useful properties. In addition, the need to have asubstantial vapor density at the expander inlet has a direct impact onthe fluid choice and the diameter of the scrolls, both of which impactscroll cost. With many fluids, the condensing temperature and need forsignificant expansion result in very high scroll inlet pressures(increasing pumping power) or super critical conditions at the inlet,resulting in difficulties in evaporator design operation and control.These same conditions are of concern when one considers other natural(hydrocarbon) fluids. For example, while pentane, butane, and propanewere all considered as potential working fluids, the inventorsdetermined that, of the naturally-occurring hydrocarbons, isopentaneoffers superior fluid properties for micro-CHP applications.

[0010] According to another aspect of the present invention, acogeneration system is disclosed. The cogeneration system includes anorganic working fluid, a heat source capable of superheating the organicworking fluid, a first circuit to transport the organic working fluid,and a generator to produce electricity. At least a portion of the firstcircuit, which includes a scroll expander, a condenser and a pump, is inthermal communication with the heat source. The pump circulates theorganic working fluid through the first circuit. Preferably, the heatsource is a burner in thermal communication with an evaporator such thatheat provided by the burner causes the organic working fluid that flowsthrough the evaporator to become superheated. In the present context,the term “thermal communication” is meant to broadly cover all instancesof thermal interchange brought about as a result of coupling betweensystem components, whereas the more narrow “heat exchange communication”(discussed below) is meant to cover the more specific relationshipbetween direct, adjacent heat exchange components designed specificallyfor that purpose. By the nature of the organic working fluid, it remainsin a superheated state from prior to entering the scroll expander toafter it exits the same. The high vapor density and heat transferproperties of the superheated organic working fluid ensure that maximumheat and power can be extracted from the fluid without having to resortto a large expander.

[0011] The cogeneration system can be configured such that the organicworking fluid is directly-fired or indirectly-fired. In the formerconfiguration, the relationship between the burner and the organicworking fluid-carrying evaporator is such that the flame from thecombustion process in the burner directly impinges on either the conduitcarrying the fluid or a container (alternately referred to as acombustion chamber) that houses at least a part of the organic workingfluid-carrying conduit such that the part of the conduit where theorganic working fluid becomes superheated is considered the evaporator.In the latter configuration, the flame from the combustion process inthe burner gives up a portion of its heat to conduit making up asecondary circuit, which in turn conveys a heat exchange fluid to aninterloop heat exchanger. The heat exchange fluid could be water, amixture of water and a freeze-inhibiting additive (such as propyleneglycol), or an organic, such as that of the organic working fluid of thefirst circuit. The first loop of the interloop heat exchanger is fluidlyconnected to the organic working fluid-conveying first circuit, whilethe second loop is fluidly connected to the heat exchangefluid-conveying second circuit. Preferably, the interloop heat exchangeris situated between the pump and the scroll expander of the firstcircuit so that it acts as an evaporator for the organic working fluid.The latter configuration may also include a space heating loop preheatdevice that is in heat exchange communication with the condenser secondloop such that a portion of the heat still present in the heat exchangefluid after giving up a portion of its heat to the organic working fluidin the interloop heat exchanger can be used to preheat fluid in anexternal SH loop.

[0012] Also, as with the former configuration, the burner can bedisposed within a container. In both configurations, the container mayinclude an exhaust duct to carry away combustion products (primarilyexhaust gas), an exhaust fan to further facilitate such product removal,as well as an exhaust gas heat exchanger disposed adjacent (preferablywithin) the exhaust duct so that residual heat present in the exhaustgas can be used for supplemental heating in other parts of thecogeneration system. The exhaust gas heat exchanger can further includean exhaust gas recirculation device to further improve heat transferfrom the exhaust gas. In the former configuration, the heat picked up bythe exhaust gas heat exchanger can be routed to various places withineither the first circuit or the space heating loop to provide additionalpreheat of the organic working fluid or space heating fluid,respectively. In addition, either configuration may be adapted toexchange heat with an external DHW loop. The heat exchange may furthertake place in a heat exchanger configured similar to the condenser, suchthat two individual loops are placed adjacent one another to facilitatethe transfer of heat between the respective fluids flowing therethrough,or in a storage tank (such as a hot water storage tank) such that thefluid stored therein (preferably water) is kept at an elevatedtemperature to have a readily-available supply of hot tap, bath andshower water. In the case of a storage tank-based approach, additionalheating of the liquid in the tank can occur by a heating element thatreceives its power from the generator. Where no tank is present, theheat to the DHW loop can be taken from a connection to the first circuitcondenser (in the directly-fired configuration) or the heat exchangefluid flowing through the second circuit (in the indirectly-firedconfiguration). Furthermore, in either of the directly-fired orindirectly-fired configurations, if it is desired to preserve theability to provide DHW while maintaining an overall simplistic, low-costsystem, an oversized or multiple-staged burner may be used. This promptheating can reduce the size of or even obviate the need for a largestorage tank while still capable of providing substantially “instant”hot water when required.

[0013] The operating conditions, including maximum temperature andpressure, of the cogeneration system's first circuit are configured tobe within the design range of the organic working fluid. A controllercan be incorporated to monitor and, if necessary, change operatingparameters within the system. Switches, sensors and valves can beincorporated into the system to help the controller carry out itsfunction. For example, to protect the expander from overspeeding duringstartup or shutdown transients, or low (or no) grid load, the controllercan direct block and bypass valves to actuate, thereby forcing thesuperheated organic working fluid to bypass the expander. The controllermay also integrate with user-determined conditions through thethermostat.

[0014] According to another aspect of the present invention, anindirectly-heated micro-CHP, including a heat source, first and secondfluid circulating loops and an interloop heat exchanger, is disclosed.The indirectly-fired micro-CHP is advantageous in terms of systemflexibility and maintainability. Multiple fluid-circulating loops areemployed such that the heat source (for example, a burner) is providedto a second fluid circulating loop that is in thermal communicationwith, but fluidly isolated from, a first fluid circulating loop. Thesecond fluid circulating loop includes piping used to convey a heatexchange fluid. This piping is preferably coiled and finned to maximizeheat transfer between the heat source and the heat exchange fluid. Atleast one pump is used to circulate the heat exchange fluid. The secondfluid circulating loop further contains a parallel set of sub-loops, oneof which passes through a DHW heat exchanger to heat up municipal water,while the other passes through the interloop heat exchanger as anintermediary between the heat source and the organic working fluidflowing through the first fluid circulating loop. In addition to passingthe organic working fluid through the interloop heat exchanger, thefirst fluid circulating loop includes a scroll expander connected to agenerator, a SH heat exchanger, and a circulation pump. Upon theapplication of heat, the organic working fluid becomes superheated, thengets expanded in the scroll expander to turn the generator, therebyproducing electrical power. The lower pressure, but still superheatedorganic working fluid leaving the scroll expander enters the SH heatexchanger, where another fluid, typically air or water, can be passedthrough and heated by the organic working fluid. This SH fluid is thencirculated to radiators or similar space heating devices within adwelling. The circulation pump returns the condensed organic workingfluid to the interloop heat exchanger, where it can repeat the process.

[0015] Optionally, a preheat device for the SH loop can be placed inheat exchange communication with the second fluid circulating loop suchthat additional SH may be effected. In addition, as with the previousaspect, the heat source may include a burner disposed within acombustion chamber-type container. The container may include an exhaustduct, an exhaust fan, and an exhaust gas heat exchanger disposedadjacent the exhaust duct. The exhaust gas heat exchanger can furtherinclude an exhaust gas recirculation device to further improve heattransfer from the exhaust gas. Residual heat that would otherwise bevented out the duct and to the atmosphere can be captured and reroutedto other parts within the system. For example, the exhaust gas heatexchanger may be integrated into the first sub-loop of the second fluidcirculating loop in order to provide additional heating to the DHW heatexchanger.

[0016] According to yet another aspect of the present invention, adirectly-fired cogeneration system configured to circulate an organicworking fluid is disclosed. The directy-fired micro-CHP is advantageousin terms of system cost and simplicity. The system includes a pipingloop that defines an organic working fluid flow path, an organic workingfluid disposed in the piping loop, an evaporator disposed in the organicworking fluid flow path, a burner in thermal communication with theevaporator such that heat transferred to the evaporator superheats theorganic working fluid, a scroll expander disposed in the organic workingfluid flow path such that the superheated organic working fluid passingthrough the scroll expander remains superheated upon discharge from thescroll expander, a generator operatively responsive to the scrollexpander to generate electricity, a condenser, and a pump disposed inthe organic working fluid flow path between the condenser and theevaporator. The condenser comprises a primary loop disposed in theorganic working fluid flow path such that the primary loop is in fluidcommunication with the scroll expander, and a secondary loop in heatexchange relationship with the primary loop, where the secondary loop isconfigured to transfer at least a portion of the heat contained in theorganic working fluid passing through the primary loop to an externalloop, such as a space heating device.

[0017] Optionally, the directly-fired micro-CHP system includes acontroller, valves, combustion chamber and exhaust features similar tothat of the previous aspects. Also, as with the previous aspects, theorganic working fluid is preferably either a naturally occurringhydrocarbon (such as isopentane) or a halocarbon refrigerant, such asR-245fa. In addition, the heat source, which can be a burner, may beoversized to provide additional heat in variations of the system that donot employ a storage tank for DHW purposes. In this situation, theburner can be either larger, or a multi-staged device such that eachstage is dedicated to a particular part of the external heatingcircuits, such as the SH or DHW circuits. Furthermore, the externalheating circuits can be coupled to the cogeneration system from a singleconnection on the condenser such that bifurcated paths corresponding tothe SH and DHW loops can both be accommodated.

[0018] According to still another aspect of the present invention, amicro combined heat and power system is disclosed. The micro combinedheat and power system comprises an electricity generating loop and aconnection to an external heating loop. The electricity generating loopincludes a burner for raising the temperature of the organic workingfluid such that the organic working fluid becomes superheated, a scrollexpander to receive the superheated vapor such that the working fluidremains in a superheated state after passing therethrough, a generatoroperatively coupled to the scroll expander to produce electricity, acondenser disposed in fluid communication with the scroll expander and apump to circulate the organic working fluid. The connection is disposedin the condenser, and is configured to place the external heating loopin thermal communication with the condenser. This external heating loopcan be either a DHW loop, an SH loop, or both. As with the previousaspects of the invention, similar controller, combustion chamber andrelated features may be incorporated.

[0019] According to an additional aspect of the present invention, asystem for the production of domestic hot water, space heat andelectricity from a Rankine-based cycle with an organic working fluid isdisclosed. The system includes a substantially closed circuit fluid pathconfigured to transport the organic working fluid therethrough, a burnerconfigured to provide sufficient heat to superheat the organic workingfluid, and a controller to regulate the operation of the system. Thesubstantially closed circuit fluid path is at least partially defined bya coiled conduit configured to act as a heat transfer element for theorganic working fluid, and includes as components a scroll expander, agenerator, a condenser and a pump. The term “tube” can be usedinterchangeably with “conduit”, as both describe a closed hollow vesselused for the transport of fluids. The burner is in thermal communicationwith the substantially closed circuit fluid path's coiled tube. Thescroll expander is configured to accept the superheated organic workingfluid. The condenser is configured to extract at least a portion of theheat remaining in the organic working fluid after the organic workingfluid passes through the scroll expander. The pump pressurizes andcirculates the organic working fluid.

[0020] According to yet an additional aspect of the present invention,an indirectly-fired cogeneration system comprising a heat source, apassive heat transfer element in thermal communication with the heatsource, a first circuit, a generator and a second circuit is disclosed.The first circuit is configured to transport an organic working fluid,and is disposed adjacent an end of the passive heat transfer elementsuch that heat transferred from the passive heat transfer elementincreases the energy content of the organic working fluid. The firstcircuit is made up of at least a scroll expander configured to receivethe organic working fluid, a condenser in fluid communication with thescroll expander, and a pump configured to circulate the organic workingfluid. The condenser is configured to transfer at least a portion of theexcess heat contained in the organic working fluid to an externalheating loop. As with the previous aspects, the generator is coupled tothe scroll expander to produce electricity in response to motionimparted to it from the scroll. The second circuit is configured totransport a heat exchange fluid therethrough, and is disposed adjacentan end of the passive heat transfer element such that heat transferredtherefrom increases the energy content of the heat exchange fluid. Thesecond circuit is made up of at least a combustion chamber disposedadjacent the heat source such that exhaust gas can be removed. Detailsrelating to the combustion chamber are similar to those discussed inconjunction with the previous aspects, with the exception that one endof the passive heat transfer element (which is preferably a heat pipe)is disposed inside the combustion chamber so that such end absorbs heatfrom the heat source.

[0021] According to still another aspect of the present invention, acogeneration system comprising a heat source, a passive heat transferelement in thermal communication with the heat source, and a firstcircuit is disclosed. The first circuit is configured to transport anorganic working fluid, and is disposed adjacent an end of the passiveheat transfer element such that heat transferred from the passive heattransfer element superheats the organic working fluid. The first circuitis made up of at least a scroll expander configured to receive theorganic working fluid, a condenser in fluid communication with thescroll expander, and a pump configured to circulate the organic workingfluid. A generator is coupled to the scroll expander to generateelectricity in response to the expansion of the organic working fluid inthe scroll. The condenser is configured to transfer at least a portionof the excess heat contained in the organic working fluid to an externalheating loop. As with the previous aspect, the passive heat transferelement is preferably a heat pipe, and its integration into thecombustion chamber is similar.

[0022] According to another aspect of the present invention, a method ofproducing heat and electrical power from a cogeneration device isdisclosed. The method includes the steps of configuring a first circuitto transport an organic working fluid, superheating the organic workingfluid with a heat source that is in thermal communication with the firstcircuit, expanding the superheated organic working fluid in a scrollexpander, turning a generator that is coupled to the scroll expander togenerate electricity, cooling the organic working fluid in a condensersuch that at least a portion of the heat in the organic working fluidpassing through the condenser is transferred to an external heatingloop, using at least a portion of the heat that has been transferred tothe external heating loop heat to provide space heat, and returning theorganic working fluid exiting the condenser to a position in the firstcircuit such that it can receive additional heat input from the heatsource.

[0023] Optionally, the method includes maintaining the organic workingfluid in a superheated state through the expanding step. As anadditional step, the method can selectively use at least a portion ofthe heat that has been transferred to the external heating loop to heata domestic hot water loop. An alternative set of steps can be used toconfigure a second circuit to transport a heat exchange fluid to a DHWloop where the DHW loop is decoupled from the SH loop that is thermallycoupled to the condenser. The second circuit is defined by a piping loopflow path that is in thermal communication with the heat source. Thesecond circuit is in heat exchange communication with at least onedomestic hot water loop, such as a heat exchanger or a water storagetank, for example. The second circuit is configured such that at least aportion of the heat that has been transferred to the heat exchange fluidwill go to heat a fluid (such as water) in the domestic hot water loop.Preferably, the organic working fluid is superheated to about 10 to 30degrees Fahrenheit above its boiling point in the superheating step, andis pressurized to a maximum pressure of about 200 to 450 pounds persquare inch in the returning (pumping) step. In addition, thesuperheating step produces a maximum temperature of between about250-350 degrees Fahrenheit in the organic working fluid. Moreover, theexpanding step is conducted such that the electrical output of thegenerator is up to 10 kilowatts, while the cooling step is conductedsuch that the thermal output transferred to the external heating loop isup to 60 kilowatts. The heat source can either directly or indirectlyfire the organic working fluid. An additional step may further includeoperating a set of valves configured to permit the organic working fluidto bypass the scroll expander upon a preset condition, which can be agrid outage, startup transient or shutdown transient.

[0024] According to another aspect of the present invention, a systemfor the production of electricity and space heat through the expansionof an organic working fluid in a superheated state is disclosed. Thesystem comprises an organic working fluid, a flow path configured totransport the organic working fluid, a combustion chamber disposed inthe flow path, a scroll expander disposed in the flow path to receiveand discharge the organic working fluid in the superheated state, agenerator operatively coupled to the scroll expander to produceelectricity, a condenser in fluid communication with the scrollexpander, and a pump to circulate the organic working fluid through theflow path. The combustion chamber comprises a burner, a heat transferelement adapted to convey the organic working fluid adjacent the burner,and an exhaust duct to convey combustion products produced by the burnerto the atmosphere. As with previous aspects, coupling between thecondenser and an external heating loop can be used to effect heatexchange with an SH loop. In addition, system regulating devices, suchas a controller, switches and valves may be employed, as can additionalheat exchange devices that couple to the exhaust duct or the condenser,also discussed in conjunction with the previous aspects.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0025] The following detailed description of the preferred embodimentsof the present invention can be best understood when read in conjunctionwith the following drawings, where like structure is indicated with likereference numerals and in which:

[0026]FIG. 1 shows a schematic diagram of an integrated micro-CHP systemaccording to an embodiment of the present invention showing anindirectly-fired configuration with a storage tank and both SH and DHWcapability;

[0027]FIG. 2 shows a schematic diagram of an integrated micro-CHPshowing an indirectly-fired configuration with no storage tank and bothSH and DHW capability;

[0028]FIG. 3 shows a schematic diagram of an integrated micro-CHPshowing a directly-fired configuration with no storage tank and both SHand DHW capability;

[0029]FIG. 4 shows a schematic diagram of an integrated micro-CHPshowing a directly-fired configuration with a storage tank and both SHand DHW capability;

[0030]FIG. 5 shows a schematic diagram of an integrated micro-CHPshowing a directly-fired configuration with no storage tank and SHcapability;

[0031]FIG. 6 shows the integration of a heat pipe into anindirectly-fired embodiment of the present invention, furtherhighlighting a common heat exchanger for both SH and DHW;

[0032]FIG. 7 shows the integration of a heat pipe into a directly-firedembodiment of the present invention, further highlighting a common heatexchanger for both SH and DHW; and

[0033]FIG. 8 shows the details of an exhaust gas heat exchanger,including details of an exhaust gas recirculation device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] Referring initially to FIG. 1, one embodiment of the micro-CHPsystem 1 is an indirectly-heated, dual-loop system that includes a first(or primary) circuit 100 and a second circuit 150. An advantage of theindirectly fired system is that first circuit boiler (or evaporator)conduit overheating and subsequent bum-out is avoided. First circuit 100includes a expander 101, a condenser 102, a pump 103 and one portion ofinterloop heat exchanger 104. An organic working fluid (such asnaturally-occurring hydrocarbons or halocarbon refrigerants, not shown)circulates through the loop defined by the fluidly-connected expander101, condenser 102, pump 103 and interloop heat exchanger 104. Piping110 is used to connect the various components of first circuit 100,whereas the pump 103 provides the pressure to supply the organic workingfluid to the interloop heat exchanger 104, thereby completing the firstcircuit 100. A generator 105 (preferably induction type) is coupled toexpander 101 such that motion imparted to it by expander 101 generateselectricity. While the expander 101 can be any type, it is preferablethat it be a scroll device. The scroll expander can be a conventionalsingle scroll device, as is known in the art. An oil pump 108 is used toprovide lubricant to the scroll. The presence of oil helps to establisha seal between the intermeshed stationary and orbiting wraps that makeup the scroll's crescent-shaped chambers (not shown). A level indicatorswitch 120 with level high 120A and level low 120B indicators is placedat the discharge of condenser 103. Controller 130 is used to regulatesystem operation. It senses parameters, such as organic working fluidtemperatures, at various points within the first circuit and levelinformation taken from the level indicator switch 120. Throughappropriate program logic, it can be used to open and close valves (notpresently shown) in response to predetermined conditions, such as anelectric grid outage. The generator 105 is preferably an asynchronousdevice, thereby promoting simple, low-cost operation of the system 1, ascomplex generator speed controls and related grid interconnections arenot required. An asynchronous generator always supplies maximum possiblepower without controls, as its torque requirement increases rapidly whengenerator 105 exceeds system frequency. The generator 105 can bedesigned to provide commercial frequency power, 50 or 60 Hz, whilestaying within close approximation (often 150 or fewer revolutions perminute (rpm)) of synchronous speed (3000 or 3600 rpm).

[0035] An external heating loop 140 (shown preferably as an SH loop) canbe coupled to first circuit 100 via connectors (not shown) on condenser102. As an option, a preheat coil 145 can be inserted into the externalheating loop 140 such that the hydronic fluid (typically water) flowingtherethrough can receive an additional temperature increase by virtue ofits heat exchange relationship with heat exchange fluid flowing throughsecond circuit 150 (discussed in more detail below). The hydronic fluidflowing through external heating loop 140, is circulated with aconventional pump 141, and is supplied as space heat via radiator 148 orrelated device. As an example, hydronic fluid could exit the condenser102 at about 50° Celsius and return to it as low as 30° Celsius. Thecapacity of the system 1 is up to 60 kW_(t); however, it is within thescope of the present invention that larger or smaller capacity unitscould be utilized as needed. Inherent in a micro-CHP (cogeneration)system is the ability to provide heat in addition to electricity. Excessheat, from both the heat source and the expanded working fluid, can betransferred to external DHW and SH loops. The nature of the heatexchange process is preferably through either counterflow heatexchangers (for either or both the DHW and SH loops), or through aconventional hot water storage tank (for a DHW loop). It will beappreciated by those of ordinary skill in the art that while theembodiments depicted in the figures show DHW and SH heat exchangers inparallel (and in some circumstances being supplied from the same heatexchange device, shown later), it is within the spirit of the presentdisclosure that series or sequential heat exchange configurations couldbe used.

[0036] Second circuit 150 includes two parallel sub-loops 150A, 150B.Heat to the two parallel sub-loops 150A, 150B is provided by a burner151, which is supplied with fuel by a gas train 152 and variable flowgas valve 153. Piping 160 (which makes up the parallel sub-loops) passesthrough a combustion chamber 154, which is where the heat from thecombustion of fuel at burner 151 is given up to the heat exchange fluid(not shown) that flows through piping 160. Piping 160, which includes afinned tube portion 161 disposed inside the combustion chamber 154,branches out into the first parallel sub-loop 150A, which transports theheat exchange fluid that has been heated in combustion chamber 154 tointerloop heat exchanger 104 in order to give up the heat to organicworking fluid flowing through first circuit 100. Block valves (notshown) could be used to regulate flow between the sub-loops; however, byidling the pump of the inactive sub-loop, significant flow in thatsub-loop is prevented without the need for additional valving. Thesecond parallel sub-loop 150B transports the heat exchange fluid to DHWheat exchanger 180 in order to heat up domestic hot water. One side ofdomestic hot water heat exchanger 180 (which can be a water storagetank) includes coil 180A configured to transport the heat exchangefluid, and another side, the shell 180B, to transport domestic hot water(not shown) from a cold water inlet 191A, past coil 180A and to DHWoutlet 191B. Typically, the cold water comes from either a well or acity/municipal water supply. Similarly, temperature sensor 171B candetect the temperature of the DHW coming out of the DHW heat exchanger180. This sensor can also be linked to a controller 130 (discussed inmore detail below). Combustion chamber 154 includes an exhaust duct 155,an exhaust gas recirculation device 156 with exhaust duct heat exchanger157, and fan 158. It will be appreciated by those skilled in the artthat although the fan 158 is preferably shown as an induced-draft fan,it could also be a forced-draft fan, if properly located relative to thecombustion chamber 154. Temperature sensor 171A is placed at thecombustion chamber 154 outlet for the second circuit 150 to measure thetemperature conditions of the heat exchange fluid, in a manner similarto that of temperature sensor 171B. Second circuit pumps 185A, 185B areused to circulate heat exchange fluid through the second circuit 150,with pump 185B circulating heat exchange fluid through DHW heater 180and pump 185A circulating heat exchange fluid through interloop heatexchanger 104. The exhaust duct heat exchanger 157 and an exhaust gasrecirculation (EGR) device 156 accept hot exhaust gas from the burner151 and recirculate it in an internal heat exchange process, therebylowering the temperature of the exhaust gas that is pulled away andvented to the atmosphere by fan 158. The heat given up by the exhaustgas in the exhaust gas heat exchanger 157 is used to provide additionalheat to other parts of the system 1. In the present figure, thisadditional heat is used to increase the temperature of the heat exchangefluid flowing in second circuit 150.

[0037] A controller 130, which could be a programmable logic controller(PLC) or conventional microcomputer (not shown), can be used to providedetailed system control. All of the pumps can be configured to bevariable-speed, and are responsive to input signals from controller 130.Upon receipt of a signal for heat, the burner 151 ignites the fuel,while the proper circulating pump 185B or 185A is energized. For DHW,flow switch 190, in conjunction with temperature sensor 171B, provideinputs to controller 130. Flow switch 190 selects DHW mode, where theDHW set point is coupled to temperature sensor 171A. The burner gas flowand DHW pump 185B flow are regulated to provide the desired temperatureat 171B according to the temperature preset by the user on the DHWthermostat (not shown).

[0038] When the system is operating, heated heat exchange fluid ismoving past sensor 171A, which is able to provide a valid signal to thecontroller 130 so the burner 151 firing rate and pump 185B flow can beadjusted for both safe operation and the needed output. However, whenthe system is just starting, the controller 130 must be given someinitialized state which can be used as a safe operating condition untilsuch time as heat exchange fluid is flowing past temperature sensor171A. It is desirable to have a minimum amount of heat exchange fluidflow during startup, so that the fluid heats up as rapidly as possible.However, some flow is needed to prevent local overheating of the fluidin the combustion chamber 154, and to provide the controller 130 with anindication that the burner 151 is indeed firing. The gas rate is set toprovide the longest possible run time for the system, consistent withmeasured outdoor temperature and rate of change of indoor temperature.Pump 185B operates to keep the combustion chamber 154 supplied with theheat exchange fluid at the factory-preset value for temperature sensor171A. When temperature sensor 171A gets to about 50% of the thermostatset point, the pump 185B speed is increased until the temperaturereading in temperature sensor 171A reaches its set point, at which timethe burner 151 and pump 185B modulate for constant values of temperaturesensors 171A and 171B. When the flow switch 190 indicates zero flow, theburner 151 and pump 185B cease operation. A small expansion tank (notshown) can be placed in the second circuit 150 to allow for differentialthermal expansion at moderately high pressures of the heat exchangefluid.

[0039] When the user desires heat, as indicated by the room thermostat(not shown) the burner 151 comes on to about 50% of its capacity to warmup system 1. Pump 185A comes on to a speed predetermined to coincidewith the flow requirements established by the initial burner firing rateand the design response of the system. The controller 130 responds tothe user demand for heat, and the owner selected set point for roomtemperature. Burner 151 firing and pump 185A flow are controlled inpart, and conventionally by room temperature and its set point, as wellas outdoor temperature (sensor not shown). The first circuit pump 103runs fast enough to keep the organic working fluid liquid level betweenlevel low 120B and level high 120A switch settings. The controller 130instructs the pump 103 to start or speed up when the organic workingfluid liquid level rises above the level 120A, and stopping when thelevel goes below level 120B, for example.

[0040] The length of finned tube portion 161 of piping 160 that isinside the combustor 154 can be minimized by carefully selecting pumps,control points, and conduit size. Referring now to FIG. 8 in conjunctionwith FIG. 1, details of the EGR device 156 for micro-CHP system 1 isshown. In essence, the EGR device 156 functions in conjunction with theexhaust duct 155 and is an integral part of exhaust gas heat exchanger157. The hot exhaust gas stream is directed axially through EGR device156, which is preferably placed between burner 151 and exhaust duct 155.An annular recirculation duct 156B, passes some of the exhaust gas in acounterflow fashion until it is reinjected at inlet 156A. The walls ofthe EGR device 156 are cooled by the heat exchange fluid that passesthrough the duct heat exchanger 157, and as a result, the recirculationgas entering at inlet plane 156A is partially cooled. This tempered gasstream leaving at plane 156B enters the second heat transfer sectiondefined by finned tube portion 161 of second circuit piping (notpresently shown), in which additional cooling of the gas occurs. In amore compact arrangement, the inner annular duct of the EGR device 156would be replaced by an array of fine tubes (not shown), each having aflow inducer for hot gas at the inlet end. While such an approach wouldinvolve the use of a larger amount of fluid, which would increase theresponse time of the system, significant benefits could be realized,including the application of the EGR device 156 to an evaporator wherean organic working fluid is used such that the fluid is never exposed tothe full temperature of the exhaust gas, and the final heat recovery isnot reduced by any form of added flue gas dilution, especially cool air.The primary benefit of the EGR device 156 is that levels of harmfulgaseous by-products (such as NO_(x)) are reduced. An additional benefitof the EGR device is that by reducing the highest temperature that thefinned tube portion 161 is exposed to, simpler components that will havelower cost yet which can attain the same long life of more costlymaterials can be used.

[0041] Referring next to FIG. 2, an alternate embodiment of theindirectly-fired micro-CHP system 2 is shown. Here, the second circuit250 does not encompass parallel sub-loops. Instead, a single loop isrouted directly from combustion chamber 254 to interloop heat exchanger204. DHW capability, which was provided by second sub-loop 150B in theembodiment shown in FIG. 1, is now integrated into the external heatingloop 240. This external loop, that services both DHW and SH, can bebifurcated after coupling to the condenser 202, with valves 247A, 247Boperating to supply SH radiators 248 or DHW heat exchanger 280 asneeded. DHW heat exchanger 280 can be either a water tank to store hotwater (as discussed in conjunction with the previous aspect), or adual-pass counterflow heat exchange device. After the fluid (typicallywater) passes through either or both SH and DHW heat exchangers, it iscirculated through heating loop 240 back to the condenser 202 to startits cycle again. Prior to entry into the condenser 202, the fluid can bepreheated by passing it thermally adjacent second circuit 250 in apreheat device 245.

[0042] Referring now to FIGS. 3 and 4, a directly-fired micro-CHP systemis shown. This system has the advantage of being simpler inconstruction, with attendant lower cost. In the present embodiment, thesystem 3 does not include a second circuit. The interloop heat exchangerof the previous embodiments, which acted as the heat source for theprevious embodiment first circuits, is replaced by a combustion chamber304, where both the burning of fuel, through gas train 352, valve 353and burner 351, and the evaporation of the organic working fluid takesplace. As with the previous embodiments, the organic working fluid issuperheated. Generator 305, as with the previous embodiments, isasynchronously tied to a load, preferably on the customer/user side ofthe electric meter, which is typically the power grid. The load on thescroll expander 301 imposed by the grid ensures that mechanical speedsin the scroll 301 are kept within its structural limits. Block valve307A and bypass valve 307B are situated in the organic working fluidflow path defined by piping 310 (of which conduit 361 is part). Thesevalves respond to a signal in controller 330 that would indicate if noload (such as a grid outage) were on the system, allowing thesuperheated vapor to bypass around the expander, thereby avoidingoverspeed of scroll 301. In this condition, the rerouted superheatedvapor is fed into the inlet of condenser 302. Under normal operatingconditions, where there is a load on the system, the superheated vaporenters the scroll expander 301, causing the orbiting involute spiralwrap to move relative to the intermeshed fixed involute spiral wrap. Asthe superheated vapor expands through the increasing volumecrescent-shaped chambers, the motion it induces in the orbiting wrap istransferred to the generator 305, via a coupled shaft or an integralrotor/stator combination on the scroll 301. Depending on the type of oilused in the system (such as whether the oil is miscible or immisciblewith regard to the organic working fluid), scroll 301 may preferablyinclude an oil pump 308 to circulate oil present in the scroll from thesuperheated vapor. The workings of the exhaust duct 355 and fan 358 aresimilar to that of the previous aspect; however, the present EGR device356 and exhaust duct heat exchanger 357, rather than providingadditional heat to a heat exchange fluid flowing through the secondcircuit 150, 250 of the previous embodiments, can be used to providesupplemental heat to various locations within the system 3. For example,additional heat can be added to the organic working fluid coming out ofpump 385, shown at point A. Similarly, it can be used to add heat to theexternal heating loop 340 at points B or C. Precise location of the heatexchange points A, B or C would be determined by the nature of theorganic working fluid and its properties. Note that DHW heat exchanger380 can be configured as a conventional dual-pass counterflow heatexchanger, or as a water storage tank, as discussed in the previousaspects. In situations where no (or a small) storage tank is being used(such as, for example, when space is at a premium), then in order toprovide fast-responding DHW, additional heat generation may be required.One approach is to use a larger or multiple-stage burner (not shown).This could provide rapid response times to the instant or near-instantdemands associated with DHW uses (such as showers, baths and hot tapwater). Referring with particularity to FIG. 4, a variation on thedirectly-fired micro-CHP of FIG. 3 is shown. In this case, the system 4specifically includes a storage tank 480. This approach allows theinclusion of DHW capability without having to resort to increased burnercapacity. In addition, power to a storage tank heating element 480C canbe provided directly off generator 405. In addition, trade-offs betweenthe size of the storage tank 480 and the size or number of burner 451can be made to best suit the functionality and packaging/volumerequirements of the system.

[0043] Referring now to FIG. 5, a directly-fired micro-CHP system 5 isshown. This represents the most simplistic system, in that it is gearedtoward the exclusive generation of electricity and SH. By not includingDHW capability, a storage tank can be avoided without sacrificing systemfunctionality or requiring augmented burner capacity. In other respects,this system is similar to that of the previous directly-firedembodiments, including operation of the heat source componentry 551, 552and 553, exhaust componentry 555, 556, 557 and 558, organic workingfluid flow path componentry 501, 502, 503, 504, 507A,B and 508,generator 505, and sensing a controlling apparatus 520, 530.

[0044] Referring now to FIGS. 6 and 7, a variation on theindirectly-fired and directly-fired cogeneration systems of the previousaspects is shown. Referring with particularity to FIG. 6, a passive heattransfer element, preferably in the form of a heat pipe 675, can bedisposed between the first circuit 600 and the second circuit 650 toeffect heat exchange between those circuits and the heat source.Referring with particularity to FIG. 7, heat pipe 775 is disposed withinthe flow path of the first circuit, which also includes scroll expander701, condenser 702 and pump 703. In either configuration, the heat pipeis an evacuated and sealed container that contains a small quantity ofworking fluid, such as water or methanol. When one end of the pipe(typically referred to as the evaporator end) is heated, the workingfluid rapidly vaporizes, due in part to the low internal pressure of thefluid. The vapor travels to the lower-pressure opposite end (typicallyreferred to as the condenser end), giving up its latent heat.Preferably, gravity or capillary action allows the condensed fluid tomove back to the evaporator end, where the cycle can be repeated. Whenthe fluid has a large heat of vaporization, a significant amount of heatcan be transferred, even when the temperature differences between theopposing ends is not great. In other regards, the operation of thesystems is similar to that of the previous aspects.

[0045] Referring now to FIG. 8, details of the exhaust duct heatexchanger 157 and the exhaust gas recirculation device 156 are shown.The combustion chamber 154 (not presently drawn to scale) encases enoughof the heat source apparatus, including burner 151) to ensure that theexhaust gas and related combustion products are entrained into theexhaust duct 155 such that they can be vented to the atmosphere. Aninduced draft fan (shown elsewhere) can be used to ensure thoroughventing of the combustion products. The exhaust gas recirculation device156 is a co-annular duct that takes the exhaust gas leaving the regionaround burner 151 through the inner annulus 156A, and doubles back aportion of the gas to flow in the outer annulus 156B. During the timethat the portion of the gas that is recirculating through the outerannulus 156B, it is giving up some of its heat to the exhaust duct heatexchanger 157, which is shown as a coiled conduit. From here, the coiledconduit of the heat exchanger 157 can be routed to other locations(shown elsewhere) in the system, where it can then be used to providesupplemental heat.

[0046] Having described the invention in detail and by reference topreferred embodiments thereof, it will be apparent that modificationsand variations are possible without departing from the scope of theinvention defined in the appended claims. More specifically, althoughsome aspects of the present invention are identified herein as preferredor particularly advantageous, it is contemplated that the presentinvention is not necessarily limited to these preferred aspects of theinvention.

We claim:
 1. An indirectly-heated micro combined heat and power systemcomprising: a heat source; an interloop heat exchanger in thermalcommunication with said heat source; a first fluid-circulating loop withat least a portion thereof passing through a first channel of saidinterloop heat exchanger, said first fluid-circulating loop comprising:an organic working fluid; a scroll expander; a generator operativelyresponsive to said scroll expander to generate electricity; a condenserin fluid communication with said scroll expander, said condenser adaptedto establish a heat exchange relationship between said organic workingfluid and an external heat exchange fluid for space heating within adwelling; and a pump for the circulation of said organic working fluid;and a second fluid circulating loop with at least a portion thereofpassing through a second channel of said interloop heat exchanger suchthat said second fluid circulating loop is in thermal communication withsaid first loop, said second fluid circulating loop comprising: a firstsub-loop comprising: piping to circulate a heat exchange fluid disposedin said second fluid-circulating loop, at least a portion of said pipingin thermal communication with said heat source; a domestic hot waterheat exchanger; and at least one pump to circulate a portion of saidheat exchange fluid through said domestic hot water heat exchanger; asecond sub-loop comprising: piping to circulate said heat exchange fluidsuch that it is in heat exchange relationship with said organic workingfluid in said interloop heat exchanger; at least one pump to circulate aportion of said heat exchange fluid through said interloop heatexchanger,  wherein said heat source, said heat exchanger, said firstloop and said scroll expander are configured such that, upon applicationof heat from said heat source to said organic working fluid via saidinterloop heat exchanger, said organic working fluid becomes superheatedto an extent that said organic working fluid remains superheated atleast through said scroll expander.
 2. An indirectly-heated microcombined heat and power system according to claim 1, further comprisingan exhaust duct in fluid communication with said heat source such thatproducts from said heat source may be removed from said micro combinedheat and power system.
 3. An indirectly-heated micro combined heat andpower system according to claim 2, further comprising a heat exchangerin thermal communication with said exhaust duct.
 4. An indirectly-heatedmicro combined heat and power system according to claim 1, furthercomprising a space heating loop preheat device placed in heat exchangecommunication with said second fluid circulating loop.
 5. Anindirectly-fired cogeneration system comprising: a heat source; apassive heat transfer element in thermal communication with said heatsource; a first circuit disposed adjacent an end of said passive heattransfer element such to accept heat transferred therefrom, said firstcircuit comprising: an organic working fluid that becomes superheatedupon receipt of heat from said passive heat transfer element; a scrollexpander configured to receive said superheated organic working fluid; acondenser in fluid communication with said scroll expander, saidcondenser configured to transfer at least a portion of the excess heatcontained in said organic working fluid to an external heating loop; anda pump configured to circulate said organic working fluid through saidfirst circuit; a generator coupled to said scroll expander to produceelectricity in response to motion imparted to it from said scrollexpander; and a second circuit configured to transport a heat exchangefluid therethrough, said second circuit in thermal communication with anend of said passive heat transfer element such that heat transferredtherefrom increases the energy content of said heat exchange fluid, saidsecond circuit comprising: a combustion chamber disposed adjacent saidheat source; at least one external loop heat exchanger; and conduit totransport said heat exchange fluid between said combustion chamber andsaid at least one external loop heat exchanger.
 6. An indirectly-firedcogeneration system according to claim 5, wherein said passive heattransfer element is a heat pipe.
 7. An indirectly-fired cogenerationsystem according to claim 5, wherein said combustion chamber is definedby: an exhaust duct in combustion communication with said heat source;an exhaust fan coupled to said exhaust duct to facilitate the removal ofexhaust gas; and an exhaust gas recirculation duct in exhaustcommunication with said combustion chamber.
 8. A cogeneration systemcomprising: a heat source; a passive heat transfer element in thermalcommunication with said heat source; a first circuit disposed adjacentan end of said passive heat transfer element such to accept heattransferred therefrom, said first circuit comprising: an organic workingfluid that becomes superheated upon receipt of heat from said passiveheat transfer element; a scroll expander configured to receive saidsuperheated organic working fluid; a condenser in fluid communicationwith said scroll expander, said condenser configured to transfer atleast a portion of the excess heat contained in said organic workingfluid to an external heating loop; and a pump configured to circulatesaid organic working fluid through said first circuit; and a generatorcoupled to said scroll expander to produce electricity in response tomotion imparted to it from said scroll expander.
 9. A cogenerationsystem according to claim 8, wherein said passive heat transfer elementis a heat pipe.
 10. An cogeneration system according to claim 8, whereinsaid combustion chamber is defined by: an exhaust duct in combustioncommunication with said heat source; an exhaust fan coupled to saidexhaust duct to facilitate the removal of exhaust gas; and an exhaustgas recirculation duct in exhaust communication with said combustionchamber.
 11. A method of producing heat and electrical power from acogeneration device, the method comprising the steps of: configuring afirst circuit to transport an organic working fluid; superheating saidorganic working fluid with a heat source that is in thermalcommunication with said first circuit; expanding said superheatedorganic working fluid in a scroll expander such that said organicworking fluid is maintained in a superheated state; turning a generatorthat is coupled to said scroll expander to generate electricity; coolingsaid organic working fluid in a condenser such that at least a portionof the heat in said organic working fluid passing through said condenseris transferred to an external heating loop; using at least a portion ofsaid heat that has been transferred to said external heating loop heatto provide space heat; returning said organic working fluid exiting saidcondenser to a position in said first circuit such that it can receiveadditional heat input from said heat source; configuring a secondcircuit to transport a heat exchange fluid, said second circuit definedby a piping loop in thermal communication with said heat source and heatexchange communication with at least one domestic hot water loop;heating said heat exchange fluid with said heat source; and using atleast a portion of said heat that has been transferred to said heatingexchange fluid to heat a fluid in said domestic hot water loop.